Method and apparatus for measuring the radiant energy reflectance of materials

ABSTRACT

To provide a method and apparatus for measuring reflectances independent of the sample temperature, the sample is alternately irradiated by hot and cold radiation which is picked up by one or more detectors.

United States Patent Keith E. Nelson Rolling Hills Estates, Calif.

Nov. 15, 1968 Oct. 12, 1971 TRW Inc.

Continuation of application Ser. No. 385,227, July 27, 1964, nowabandoned.

[72] lnventor [2 l Appl. No. [22] Filed [45] Patented [73] Assignee [54]METHOD AND APPARATUS FOR MEASURING TIIE RADIANT ENERGY REFLECTANCE OFMATERIALS 10 Claims, 4 Drawing Figs.

[52] US. Cl. 250/83.3 H,

2O COMPARISON I CIRCUIT 22 51 Iliad] ..G0ln 21 22, GOln 21/48 50 Field61 Search 356/51, 188, 212; 250/226, 83.3 IR, 83.3 n

[56] References Cited UNITED STATES PATENTS 2,046,714 7/1936 Wilson6:61. 250/226 3,089,382 5/1963 116cm et a1. 356/188 3,273,448 9/1966Kelly 356/188 Primary Examiner-Ronald L. Wibert AssistantExaminer-Orville B. Chew, ll Attorney-Daniel T. Anderson ABSTRACT: Toprovide a method and apparatus for measuring reflectances independent ofthe sample temperature, the

sample is alternately irradiated by hot and cold radiation which ispicked up by one or more detectors.

PAIENTEIIBET I 2 \QTI D. C. VOLTMETER Fig. 4.

MECH. RECTIFIER (chopper) A. C. AMPLIFIER DETECTOR CIRCUIT COMPARISON ACor DC Keith E. NeIson,

INVENTOR.

AGENT.

METHOD AND APPARATUS FOR MEASURING 'IIIE RADIANT ENERGY REFLECTANCEOFMATERIALS This application is a continuation of U.S. Pat. applicationNo. 385,227 filed on July 27,1964and assigned to-the assignee of thisinvention, now abandoned.

This invention relates to a method and apparatus for measuring thereflectance of materials, and more particularly to an apparatus whichmeasures the reflectance independent of sample temperature whereby anoutput is provided which directly indicates the reflectance withoutrequiring a separate temperature measurement and computation of thereflectance indirectly. Since the emittance has a simple relationshipwith the reflectance, (i.e. p=lfor diffuse samples) the value of theemittance may be readily obtained from this relationship.

Apparatus previously used for obtaining the reflectance or emittance ofsample materials have utilized a detector, some of which are cooled, ora thermoelectric junction. Both the detector and the thermoelectricjunction make a direct measurement of the emitted plus reflected energy.However, in order to utilize the output from the detector, it isnecessary to make a temperature measurement of the sample simultaneouslyand calculate the emissivity utilizing the mathematical relationship.

Briefly stated, one preferred embodiment of the present inventionconsists essentially of a cylindrical body which is divided into twosemicylindrical cavities which are separated by an insulating partitionand maintained at two different temperatures. The semicylindricalcavities are provided with arcuate openings at opposite ends thereof,and the sample is placed adjacent the arcuate opening at one'end and oneor more detectors are positioned to receive radiation from the sampleand cavity through the arcuate slots'at the opposite end. The cylinderis rotated at a constant speed'and the output from the detector issupplied to an electronic circuit which provides an output proportionalto the reflectance of the sample material.

One object of the present invention is to provide a method and apparatusfor measuring reflectance-independent of the sample temperature, andwherein the temperatures within the instrument itself need not be known.

Another object of the present inventionis'to'provide a small portableinstrument for measuring the reflectance and/or emittance of materialsat room temperature ina simple and rapid manner, in order to meet suchrequirements'as quality control of materials, material selection andsorting functions, and which may be used for total radiation or formonochromatic radiation measurements.

Other objects and many of the attendantadvantages of this invention willbe readily appreciated, as the same becomes better understood byreference to the following detailed description, when considered withthe accompanying drawings, wherein:

FIG. 1 is a schematic view illustrating one preferred embodiment of thepresent invention which utilizes two detectors;

FIG. 2 is a longitudinal sectional view illustrating another preferredembodiment of the present inventionwhich utilizes a single detector;

FIG. 3 is an end view with portions broken away to illustrate thearrangement of the cylindrical rotor forming the hot and cold cavitieswith the drive motor and synchronized chopper for providing the outputsignal; and

FIG. 4 is a block diagram illustrating one preferred form of the outputcircuit to provide a DC voltage reading proportional to the reflectanceof the sample material to be used in conjunction with the modificationillustrated in FIGS. 2 and 3.

MODIFICATION I Referring now to the drawings in detail and moreparticularly to FIG. 1, a cylindrical rotor 11 is mounted on a shaft 12for rotation by a motor (not shown). V

The cylinder 11 is divided by the insulating partition 13 into twocavities 14 and 15. While the cavities I4 and 15 are identical in allrespects and are preferably coated with a flat black coating, one of thetwo cavities is provided with some means, such as a resistance heatingelement in the cavity 15 for raising its temperature, so that the cavity15 becomes the hot cavity which is maintained at a higher temperaturethan the cold cavity 14. While the cavity temperatures must bedifferent, it is not necessary to the satisfactory operation of theapparatus that the temperatures be known.

Thesample of material 17 may be placed on a flat surface opposite orclosely adjacent the position of the two arcuate slots 19 and 21 in oneend of the cylinder II, as the cylinder is rotated. However, if it isdesired to measure the reflectance of a large area, such as the surface18, the instrument may be portable and can be positioned adjacentthisxsurface to measure the reflectance of the particular material orcoating on the surface.

The mirrors 22 and 23 are focused on the detectors 24 and 25respectively. The radiation received from the sample 17 through theslots 19 and 21 is focused by the mirror 22 on the detector 24, and theradiation from the inside of the cavities l4 and 15 is focused by themirror 23 on the detector 25.

The radiation is received from the sample 17 and from the walls of thecavities 14 and H5 through the narrow arcuate slots 26 and 27 formed inthe end wall of cylinder 11 opposite the wider arcuate slots 19 and 21.g s

' OPERATION, MOD I Consider a sample at temperature T irradiated withintensity G. The radiant energy leaving the sample surface will be thesum of the emitted and reflected energy. The output of the detector 24viewing the sample will be proportional to this sum.

.lp pm 1. where r V, voltage output of detector 24 K, proportionalityconstant p= sample reflectance G= surrounds irradiation on sample 6,emittance of sample o'= Stephen Boltzman constant T, sample temperatureIf the sample is alternately irradiated with different intensity G and Gand the sample temperature essentially does not change, the alternatingoutput from the detector 24 will be:

The sample reflectance can be detennined from equation 2 if G and G, areknown. If the detector 25 views cavities l4 and 15 only, its output willbe proportional to the irradiation G or G, The alternating output of thedetector 25, when the irradiation is alternately G and 6, will be V,voltage output of detector 25 K, proportionally constant If detector 24views the sample while detector 25'views the surrounds equations'2 and 3may be combined to solve for the sample reflectance p=( V /K [G G,])=V,K,IV,K, 4.

Since K and K, are constants they may be combined The sample reflectanceis therefore measured by taking the ratio of the two detector voltageoutputs. The analysis was made on a total energy basis. However, theanalysis on a monochromatic basis would be the same. By introducing amonochro'mator or appropriate filters, this method may be used forspectral measurements.

The two semicircular cavities l4 and I5 separated by insulation 13 aremaintained at different temperatures T, and T, Detectors 24 and 25 viewthe sample 17 and cavity wall respectively through the small slits 26and 27 at the top edge of the cavities. As the cavities l4 and 15 rotatethe sample area viewed by detector 24 is alternately irradiated by acavity at temperature T and T, thus causing the sample 17 to beirradiated by a cavity at intensity G and G, Detector 25 alternatelyviews the cavities l4 and 25 giving a single proportional to thedifference in sample irradiation G and G The sample reflectance may thenbe determined by equation 5. Suitable electronic circuitry indicated bythe comparison circuit 20 in FIG. 1 including a summing amplifier orratio detector may be used to provide an output voltage proportional tothe reflectance of the sample.

MODIFICATION II One preferred modification of the present inventionillustrated in FIGS. 2 and 3, requires only one detector and opticalsystem and overcomes the disadvantage of the device shown in FIG. 1,which requires careful matching of the two detectors 24 and 25 foroptimum performance, and also requires a complex electronic circuit forcombining the output of the two detectors 24 and 25 to provide a signaloutput corresponding to the reflectance in accordance with the ratio ofequation 5.

In the modification of FIGS. 2 and 3, the reflectancemeasuring'instrument is provided with a housing 31 which is preferablycoated with flat black coating to eliminate the reflection of any strayradiation within the housing. A cylindrical rotor 32 is divided intocavities 33 and 34, which are preferably made from copper or somematerial having a high thermal conductance, and are mounted for rotationon a central shaft 35.

The cavity 33 is provided with a heating element 36 bonded or otherwisesecured to its entire outer surface. However, if desired both cavitiesmay be provided with a heating element, so that either cavity may beutilized as a hot cavity, but only one heater is used in any case.

The heating element 36 is provided with DC or AC power through a set ofcarbon brushes 37 and 38 connected to the heating element 36, andanother set of carbon brushes 41 and 42 leading from a source of DC orAC power to the rotary contacts 43 and 44 mounted on the shaft 35.

The shaft 35 is driven through a sprocket 45 by means of a chain drive46 from a motor 47. Chain 46 also engages a sprocket 48 to drive amechanical rectifier or chopper 49, which essentially consists of a setof contact points to phase the cavity rotation and the operation of theamplifier. This will be described in more detail in conjunction with theblock diagram of FIG. 4.

In this modification, the single detector 51 is utilized which may be avacuum thermocouple with a potassium bromide window, or other types ofradiation detector may be used depending on the particular application.

OPERATION, MOD II The operation of the instrument illustrated in FIGS. 2and 3 is somewhat similar to the operation of the device shown in FIG.I, however, the electronic circuit illustrated in block diagram form inFIG. 4 is somewhat simpler than that required for a signal output fromthe device of FIG. 1.

The signal from the thermocouple detector 51 is amplified using a narrowband AC amplifier 62 which may operate at any suitable frequency such as13 cycles/second.

The signal from the AC amplifier 62 goes through the mechanicalrectifier or chopper points 49 to a DC voltmeter 63 which may beadjusted to provide an output proportional to the reflectance of thematerial.

The system is operated by supplying heater power one of the rotatingcavities such as the cavity 33. This raises the temperature of the hotcavity 33 to a higher temperature such as 1 10 F and the cold cavity 34will be heated slightly by convection and conduction to a temperature ofapproximately 90 F. After equilibrium is reached between the twocavities 33 and 34, a zero reading is set by placing a cone, paintedinternally with flat black paint over the viewing port 59. Similarly a Ipercent reading is set using a highly reflective sample, such as ahighly polished aluminum material, which is used to set the instrumentat a particular scale reading, such as 97.5 percent depending on theparticular material used. These settings are made using the zero andgain control adjustments of the AC amplifier 62.

The mathematical analysis given above with respect to the modificationFIG. 1 also applies to the instrument of FIGS. 2 and 3. However, thesetting of a zero and percent scale reading, by placing the black coneand polished aluminum over the viewing port 58, in effect makes the termV,=l in the equation 5 above, since the energy reflected by the polishedaluminum is substantially equal to the energy emitted by the cavitywalls, and therefore the reading of the DC voltmeter 63 will indicatethe variation of V, which will then be proportional to the reflectanceof the sample.

Obviously many other modifications and variations the present inventionmay be made within the scope of the following claims.

I claim:

1. A method for measuring the reflectance of materials comprising:

A. providing a hot source and a cold source of radiation;

B. alternately exposing a sample of material to said hot and coldsources;

C. detecting the radiation reflected by said material alternately fromsaid hot and cold sources, and producing an AC signal correspondingthereto;

D. comparing the AC signals and converting said AC signal to an outputproportional to the reflectance of said material.

2. The method of measuring the reflectance of materials comprising:

A. detecting the radiant energy reflected by the material from arelatively hot and cold source alternately, and providing an AC signaltherefrom;

B. detecting the radiant energy emitted by the hot and cold sourcesalternately, and providing an AC signal therefrom;

C. comparing the AC signals from the detected energy reflected by thematerial and emitted by the sources and providing an output therefromproportional to the reflectance of the material.

3. The method of measuring the reflectance of materials comprising:

A. detecting the radiant energy reflected by the material alternately,from a relatively hot and cold cavity and providing an AC signaltherefrom;

B. detecting the radiant energy emitted alternately by said hot and coldcavities, and providing an AC signal therefrom;

C. comparing the AC signals from the detected energy reflected by thematerial and emitted by said cavities and providing an output therefromproportional to the reflectance of the material.

4. An instrument for measuring the reflectance of materials comprising:

A. a rotor having a hot cavity and a cold cavity;

B. each of said cavities having a slot at one end thereof and a secondslot at the opposite end thereof;

C. a radiation detector positioned to receive radiant energy reflectedfrom a sample through both of said slots in alternate cavitiessequentially;

D. electrical means for converting the output of said detector into anoutput signal proportional to the reflectance of a material positionedadjacent one of said slots in said cavities during rotational movementthereof.

5. An instrument for measuring the reflectance of materials comprising:

A. a rotor having a hot cavity and a cold cavity;

B. each of said cavities having a slot at one end thereof and a secondslot at the opposite end thereof;

C. a radiation detector positioned to receive radiant energy reflectedfrom a sample through both of said slots in alternate cavitiessequentially;

D. electrical means for converting the output of said detector into anoutput signal proportional to the reflectance of a material positionedadjacent one of said slots in said cavities during rotational movementthereof;

E. said electrical means comprising an AC amplifier connected to saiddetector and a mechanical rectifier connected to said amplifier anddriven at a speed correspond ing to the speed of rotation of saidcavities, and a DC voltmeter connected to said mechanical rectifier.

6. An instrument for measuring the reflectance of materials comprising:

A. a cylindrical rotor having a hot cavity and a cold cavity;

B. each of said cavities having an arcuate slot at one end thereof and asecond arcuate slot at the opposite end thereof;

C. at least one radiation detector positioned to receive radiant energyreflected from a sample through one of said arcuate slots and thenthrough the opposite arcuate slot of each of said cavities sequentially;

D. electrical means for converting the output of said detector into anoutput signal proportional to the reflectance of a material positionedadjacent said one arcuate slot in said cavities during rotationalmovement thereof.

7. An instrument for measuring the reflectance of materials comprising:

A. a cylindrical rotor having a hot cavity and a cold cavity;

B. each of said cavities having an arcuate slot at one end thereof and asecond arcuate slot at the opposite end thereof;

C. at least one radiation detector positioned to receive radiationenergy reflected from a sample through one of said arcuate slots andthen through the opposite arcuate slot of each of said cavitiessequentially;

D. electrical means for converting the output of said detector into anoutput signal proportional to the reflectance of a material positionedadjacent said one arcuate slot in said cavities during rotationalmovement thereof;

E. said electrical means comprising an AC amplifier connected to saiddetector and a mechanical rectifier connected to said amplifier anddriven at a speed corresponding to the speed of rotation of saidcavities, and a DC voltmeter connected to said mechanical rectifier.

8. An instrument for measuring the reflectance of materials comprising:

A. a rotor having a hot cavity and a cold cavity;

B. each of said cavities having a slot at one end thereof and a secondslot at the opposite end thereof;

C. one radiation detector positioned to receive radiant energy reflectedfrom a sample through one of said slots and then through the oppositeslot of each of said cavities sequentially;

D. a second radiation detector positioned to receive radiant energyemitted from the walls of said cavities through said opposite slots ofeach of said cavities sequentially;

E. electrical means for converting the output of said detectors into anoutput signal proportional to the reflectance of a material positionedadjacent said one slot in said cavities during rotational movementthereof.

9. An instrument for measuring the reflectance of materials comprising:i

A. a cylindrical rotor having a hot cavity and a cold cavity;

B. each of said cavities having an arcuate slot at one end thereof and asecond arcuate slot at the opposite end thereof;

C. one radiation detector positioned to receive radiant energy reflectedfrom a sample through one of said arcuate slots and then through theopposite arcuate slot of each of said cavities sequentially;

D. a second radiation detector positioned to receive radiant energyemitted from the walls of said cavities through said opposite arcuateslots of each of said cavities sequentrall E. eleci trical means forconverting the output of said detectors into an output signalproportional to the reflectance of a material positioned adjacent saidone arcuate slot in said cavities during rotational movement thereof.

10. Means for sequentially irradiating a sample with relatively hot andcold radiation, said means including first and second cavities eachhaving at least one radiation transparent aperture therein, said firstcavity including means for heating it to an elevated temperature withrespect to said second caviyr means for alternately exposing said sampleto radiation passing through said cavity apertures, detector means forereceiving radiant energy from said sample and said cavities in a timedrelation with said means for alternately exposing said sample, saidtimed relation being such that said detector means receives radiationfrom the sample while the sample is being exposed to radiation from oneof said cavities, and for providing electrical signals in responsethereto, and 1 means coupled to said detector means for coupling saidsignals and for providing an output proportional to the reflectance ofsaid sample. I

1. A method for measuring the reflectance of materials comprising: A.providing a hot source and a cold source of radiation; B. alternatelyexposing a sample of material to said hot and cold sources; C. detectingthe radiation reflected by said material alternately from said hot andcold sources, and producing an AC signal corresponding thereto; D.comparing the AC signals and converting said AC signal to an outputproportional to the reflectance of said material.
 2. The method ofmeasuring the reflectance of materials comprising: A. detecting theradiant energy reflected by the material from a relatively hot and coldsource alternately, and providing an AC signal therefrom; B. detectingthe radiant energy emitted by the hot and cold sources alternately, andproviding an AC signal therefrom; C. comparing the AC signals from thedetected energy reflected by the material and emitted by the sources andproviding an output therefrom proportional to the reflectance of thematerial.
 3. The method of measuring the reflectance of materialscomprising: A. detecting the radiant energy reflected by the materialalternately, from a relatively hot and cold cavity and providing an ACsignal therefrom; B. detecting the radiant energy emitted alternately bysaid hot and cold cavities, and providing an AC signal therefrom; C.comparing the AC signals from the detected energy reflected by thematerial and emitted by said cavities and providing an output therefromproportional to the reflectance of the material.
 4. An instrument formeasuring the reflectance of materials comprising: A. a rotor having ahot cavity and a cold cavity; B. each of said cavities having a slot Atone end thereof and a second slot at the opposite end thereof; C. aradiation detector positioned to receive radiant energy reflected from asample through both of said slots in alternate cavities sequentially; D.electrical means for converting the output of said detector into anoutput signal proportional to the reflectance of a material positionedadjacent one of said slots in said cavities during rotational movementthereof.
 5. An instrument for measuring the reflectance of materialscomprising: A. a rotor having a hot cavity and a cold cavity; B. each ofsaid cavities having a slot at one end thereof and a second slot at theopposite end thereof; C. a radiation detector positioned to receiveradiant energy reflected from a sample through both of said slots inalternate cavities sequentially; D. electrical means for converting theoutput of said detector into an output signal proportional to thereflectance of a material positioned adjacent one of said slots in saidcavities during rotational movement thereof; E. said electrical meanscomprising an AC amplifier connected to said detector and a mechanicalrectifier connected to said amplifier and driven at a speedcorresponding to the speed of rotation of said cavities, and a DCvoltmeter connected to said mechanical rectifier.
 6. An instrument formeasuring the reflectance of materials comprising: A. a cylindricalrotor having a hot cavity and a cold cavity; B. each of said cavitieshaving an arcuate slot at one end thereof and a second arcuate slot atthe opposite end thereof; C. at least one radiation detector positionedto receive radiant energy reflected from a sample through one of saidarcuate slots and then through the opposite arcuate slot of each of saidcavities sequentially; D. electrical means for converting the output ofsaid detector into an output signal proportional to the reflectance of amaterial positioned adjacent said one arcuate slot in said cavitiesduring rotational movement thereof.
 7. An instrument for measuring thereflectance of materials comprising: A. a cylindrical rotor having a hotcavity and a cold cavity; B. each of said cavities having an arcuateslot at one end thereof and a second arcuate slot at the opposite endthereof; C. at least one radiation detector positioned to receiveradiation energy reflected from a sample through one of said arcuateslots and then through the opposite arcuate slot of each of saidcavities sequentially; D. electrical means for converting the output ofsaid detector into an output signal proportional to the reflectance of amaterial positioned adjacent said one arcuate slot in said cavitiesduring rotational movement thereof; E. said electrical means comprisingan AC amplifier connected to said detector and a mechanical rectifierconnected to said amplifier and driven at a speed corresponding to thespeed of rotation of said cavities, and a DC voltmeter connected to saidmechanical rectifier.
 8. An instrument for measuring the reflectance ofmaterials comprising: A. a rotor having a hot cavity and a cold cavity;B. each of said cavities having a slot at one end thereof and a secondslot at the opposite end thereof; C. one radiation detector positionedto receive radiant energy reflected from a sample through one of saidslots and then through the opposite slot of each of said cavitiessequentially; D. a second radiation detector positioned to receiveradiant energy emitted from the walls of said cavities through saidopposite slots of each of said cavities sequentially; E. electricalmeans for converting the output of said detectors into an output signalproportional to the reflectance of a material positioned adjacent saidone slot in said cavities during rotational movement thereof.
 9. Aninstrument for measuring the reflectance of materials comprising: A. acylindRical rotor having a hot cavity and a cold cavity; B. each of saidcavities having an arcuate slot at one end thereof and a second arcuateslot at the opposite end thereof; C. one radiation detector positionedto receive radiant energy reflected from a sample through one of saidarcuate slots and then through the opposite arcuate slot of each of saidcavities sequentially; D. a second radiation detector positioned toreceive radiant energy emitted from the walls of said cavities throughsaid opposite arcuate slots of each of said cavities sequentially; E.electrical means for converting the output of said detectors into anoutput signal proportional to the reflectance of a material positionedadjacent said one arcuate slot in said cavities during rotationalmovement thereof.
 10. Means for sequentially irradiating a sample withrelatively hot and cold radiation, said means including first and secondcavities each having at least one radiation transparent aperturetherein, said first cavity including means for heating it to an elevatedtemperature with respect to said second cavity, means for alternatelyexposing said sample to radiation passing through said cavity apertures,detector means fore receiving radiant energy from said sample and saidcavities in a timed relation with said means for alternately exposingsaid sample, said timed relation being such that said detector meansreceives radiation from the sample while the sample is being exposed toradiation from one of said cavities, and for providing electricalsignals in response thereto, and means coupled to said detector meansfor coupling said signals and for providing an output proportional tothe reflectance of said sample.